Such cassettes are essentially used to store surplus lengths that are left on fibers for connection purposes, and to protect coils of said surplus lengths and the connections or "splices" between the optical fibers. Each cassette is assigned to connecting together two fibers, or preferably to connecting a plurality of fibers respectively to a plurality of other fibers.
Each cassette must also ensure that the radius of curvature of the surplus lengths does not drop below a minimum, either where they are coiled inside the cassette or where they enter the connections. The cassette must also make it easy to handle the coiled surplus lengths and the connections, for the purpose of installing them in the cassette and for any subsequent maintenance.
Document FR-A-2 646 928 describes such a cassette used in a connection module for connecting together optical fiber cables. That cassette includes a bottom surrounded by a discontinuous peripheral rim. Accesses for the fibers are thus delimited in the rim. In that method of using the cassette, a cover closes both the cassette and the module at the same time.
The bottom of that cassette delimits a connection support at a location provided on the bottom for that purpose substantially in a first half of the middle portion of the bottom. The support serves as a housing and retains the connections between the fibers received in the cassette. The support is formed with ribs on its bottom, which ribs delimit said location. Two of the ribs that face each other have corresponding slots in which the connections are retained.
The bottom also delimits a projecting cylinder substantially in the other half of its middle portion. The fibers are coiled around the cylinder which guides them and returns them with no less than the minimum required radius of curvature.
That cassette further includes tabs folded inwards on the rim. The coiled surplus lengths are retained underneath the tabs between the cylinder and the rim.
That cassette is used in such a way that the fibers that are received and coiled in the cassette are bare fibers. In other ways of using cassettes which operate in analogous manner to the preceding cassette, the received fibers may also be bare, or they may be protected in individual sheathing tubes, but they do not need coiled surplus lengths to be guided and returned by the above-mentioned cylinder. Such a cylinder then clutters the surface of the bottom unnecessarily, and is a hindrance when the fibers and the connections are being handled.
In the cassette described in that document, the connection support delimited on the bottom can also only be used with fiber connections that are adapted to that support. However, different types of connection may be used with dimensional and/or mechanical characteristics that differ from one type to another. The following existing different types of connection are given by way of example: connections sold under the trademark "Fibrlok" by 3M and connections sold under the trademark "Placoptic" by Alliance Technique Industrielle, having protective sheaths that are more or less rigid. In a variant the connections can be performed by welding the fibers together and protecting each weld with a flexible sheath. The different types of connection may be used selectively depending on how the cassettes are used, and on the different requirements with respect to their specific characteristics. Different supports correspond to the different types of connection.
The requirements relating to coiling and connecting optical fibers inside cassettes lead to specific cassettes being made, and as a result have repercussions on manufacturing costs and methods.